Since a vehicle has come to employ many electric devices for travelling lately, an importance of an in-vehicle power source is increasing more and more. Needs on the in-vehicle power source have been limited to perform such functions of starting an engine, operating an air-conditioner and lighting lamps in the past 20 to 30 years. In contrary to that, bi-wiring has spread and even parts of a safety system typified by an electronic parking brake (EPB) have come to be controlled by electricity. Still more, as a measure for improving gas mileage in an effort of saving energy and controlling emission of CO2, it is demanded to provide an idling stop function in stopping for a short period of time at an intersection and the like and to assure its restarting capability. Thus, a variety of functions is demanded on the power source and the battery and corresponding to that, it is desired to improve battery state detecting accuracy.
In such a circumstance, it is an important technology to accurately detect a residual capacity (SOC: state of charge) of the battery in particular for safe and comfortable driving of the vehicle and for realizing an automobile society In which an environment is taken into account, because it is linked with stable operations of the electric devices such as the EPB.
Under a battery stable condition, open circuit voltage (OCV) and the SOC thereof have a relationship of correspondence of one-to-one in general (a reference numeral 81 in FIG. 12). However, after discharging and charging electricity, the battery is affected respectively by ion generating and annihilating reactions on a surface of a polar plate due to electro-chemical reactions and by moves of ions due to diffusion and convection of electrolytic solution. Therefore, it takes a time, e.g., around 20 hours, to converge to the stable OCV and the OCV does not correspond one-to-one with the SOC when there is such temporal change. FIGS. 13 and 14 are graphs showing examples of temporal change of the OCV when the SOC and temperature of the battery are constant. FIG. 13 shows that it takes a time until when the OCV (reference numeral 82) is stabilized to a constant value even if the SOC is constant. While FIG. 14 shows changes of the OCVs (reference numerals 83, 84, 85) of batteries whose states of health (SOH) are different, it indicates that the OCVs do not converge to the identical OCV if the SOHs are different even if the SOC and temperature are adjusted so as to fall under the same condition and even if the latest discharge/charge conditions are kept same.
As described above, the influence of the SOH is not reflected in finding the SOC from the OCV by using the state detecting method just by utilizing the latest discharge/charge history. Then, there is a problem that accuracy of state detection drops if the SOC is found from the OVC without reflecting the battery depletion condition.
Patent Document 1 (Japanese Patent Application Laid-open No. Hei.2005-106615 gazette) is known as one exemplary prior art. The Document 1 uses transient response corresponding to discharge/charge history to compensate the OCV as a method for detecting the OCV and SOC of a secondary battery. The Document 1 also refers to the transient response changes corresponding to discharge/charge time and to a resistant component, a polarization component corresponding to internal reaction of the battery and diffusion speed of electrolytic solution.
According to the detection method described in Patent Document 1, it is possible to find depletion corresponding to a fast reaction speed within the battery by measuring a short-period transient response by discharging and charging for a short period of time. However it is unable to measure a long-term transient response, so that it is unable to detect depletion caused by slow reaction speed. Although it is necessary to discharge and charge for a long period of time to measure the long-term transient response, the SOC changes as the discharge/charge time becomes longer and hence the short-term transient response thereafter also changes. Thus, it is unable to detect the depletion corresponding to the different reaction speeds within the battery and to detect the states of the battery such as the residual capacity by the detecting method described in Patent Document 1.
Not only the ion generating and annihilating reactions (fast reaction speed) caused in the vicinity of the polar plate but also ion diffusion speed within the electrolytic solution (slow reaction speed) affects in a series of reaction processes called an electrical-chemical reaction within the battery. The reaction processes whose speeds are different significantly affect the accuracy of the state detection as a factor of error in such reaction system.